This invention relates generally to static dissipative devices, and more particularly to a static dissipative wrist strap which simultaneously grounds the body of an operator and his/her static electricity generating garment.
In industry today, certain sensitive electronic components, such as integrated circuits and associated components are extremely susceptible to damage from static electricity. While the human body cannot even feel static electricity at levels less than 3,000 volts, a static electricity charge of as little as 100 volts can damage or even destroy sensitive electronic components. In the industrial environment, it is well known that static electricity can be generated or built up in two ways, i.e. in the human body itself, or in certain garments, especially those made from wool, silk, and synthetic fibers. Even cotton, in low humidity situations, can cause static build up of sufficient levels to cause damage or destruction to sensitive electronic components.
The cost of damage and rework on electronic components and circuits due to the discharge of static electricity has been estimated at five hundred million dollars per year by some experts. Other experts believe an annual potential of five billion dollars can be saved by the elimination of static-related damage. If damage as a result of static electricity is discovered at the chip stage, it is relatively inexpensive to replace. If it is discovered at the board stage (assembled chips) the cost may be from $15-$30. If the damage is not discovered until the defective component is incorporated into a piece of machinery, the cost can run into thousands of dollars, or even worse can lead to product failure.
Awareness and recognition of the scope of the potential damage from static electricity is a large part of the problem. While an employee cannot feel a 1,000 volt static discharge, or see it, hear it, smell it, or taste it, such a relatively undetectable static discharge can cause a very large number of electronic component failures. The attack on this problem must be two-fold, as both the operator and the garments which he wears must be grounded. As far as the garments are concerned, there have been some attempts to apply topical treatments onto laboratory coats or jackets to dissipate static electricity. Such treatments are not completely satisfactory because the treatment may be easily lost by laundering, and it is not believed that topical treatments can effectively maintain a conductivity such the resulting resistance of the fabric is low. Recently, conductive fabrics have been developed which are woven from a blend of fibers containing polyester, cotton, and steel. This fabric is classed as static dissipative and can be grounded to effectively dissipate static electricity and prevent damage.
Further, conductive wrist straps have been developed, such as those illustrated and described in U.S. Pat. Nos. 4,398,277; 4,459,633; and 4,475,141. Such wrist or ankle straps are of a conductive material and drain the body's static electricity to ground. The aforementioned static dissipative apparel has, in general, been a parallel system which is not positively grounded (by grounding wires) unless the garment is in contact with a grounding device such as a properly grounded conductive chair.
Recently the practice of snapping the cuff of the static dissipative garment to a complementary snap on the wrist band has been introduced and is being used somewhat in industry today. The disadvantage here is that the snapping of the cuff directly to the wrist band restricts the movement of the sleeve and the movement of the individual.
The present invention provides an elasticized conductive tab between the wrist band and garment which grounds the garment as is necessary through the same grounding mechanism as that which grounds the operator. This approach allows for greater freedom of movement by the wearer.
Therefore, in accordance with the present invention, in general there is provided a conductive body strap for simultaneously electrically connecting both the operator and an overlying garment to electrical ground. The body strap includes a stretchable band of elastomeric fabric adapted to elastically embrace a body member such as the wrist, ankle, waist, or otherwise in a closed loop. The stretchable band has an inner and outer surface with one or more conductive yarns extending along at least the inner surface in electrical contact with the body member which it encircles. A tab formed of an elastomeric fabric extends outwardly from the closed loop and also includes one or more conductive yarns therein connected to the conductive yarns in the closed loop. A first coupling means, such as a snap adjacent free end the of the tab, electrically connects the conductive yarns in the tab to the conductive yarns in the garment, and a second coupling means electrically connects the conductive yarns in the band through an electrical cable to ground.
In the more preferred embodiments, both the body strap and extension are formed of a single length of the same type of conductive, elastomeric fabric with the first coupling means at one end thereof. The other end of the fabric length is formed into a loop and attached to an intermediate point to form the band. All couplings may be metallic snap systems.
It is therefore an object of the present invention to provide a conductive body strap which simultaneously electrically connects the body of an operator and the garment which he is wearing to electrical ground.
Another object of the invention is to provide a conductive body strap of the type described which includes an elastomeric conductive strap encircling the wrist or ankle and includes an elastic extension which is electrically connected to the cuff of an overlying garment.